Such electronic circuits as, for instance, emitter- and source-coupled transistor pairs are widely used, typically as differential amplifiers. The widespread use of these circuits arises, among other things, from the fact that differences between input signals can be amplified without their individual magnitudes being relevant. Further, the operation of the differential circuit strongly depends on the mutual matching of its components, specifically its transistors. The typical advantages of transistors fabricated in a monolithic technique render the differential pair particularly suitable for use in an integrated circuit. Another reason for the widespread use of such differential pairs is due to their ability of being cascaded directly without interstage coupling capacitances.
This kind of differential amplifier is biased by a current source at the tail node coupling the main current paths of the active devices, e.g. transistors. FIG. 1 shows an example of a differential amplifier in its simplest form. The amplifier comprises npn bipolar transistors 12 and 14, that have their emitters coupled to each other and to tail node 16, inputs 18 and 20, and outputs 22 and 24. The current source is constituted by a resistor 26 arranged between tail node 16 and a power supply node 28, the other supply node not being shown.
The compliance of such a circuit, i.e., the minimum power supply voltage to be applied to the circuit to make this particular circuit function correctly equals the sum of the voltage drop across the resistor 26 and the voltage drop across the main current path of one of the active devices, i.e. the collector - emitter path of transistor 12 or 14. This compliance value is generally the lowest that can be achieved for an amplifier/current source combination. The value of the bias current is determined by the value of this resistor 26 and by the value of the DC voltages at inputs 18 and 20. In this example the value of the bias current depends on the transistor-characteristics and therefore on temperature, supply voltage and the dc voltages at inputs 18 and 20. This causes the bias current to deviate from its nominal value. Since accurate setting of the bias current is required in many circuits, such deviation is undesirable.
FIG. 2 shows a known implementation of a current source that alleviates these problems. In FIG. 2 a main current path of a control transistor 30 is arranged between tail node 16 and resistor 26. Control transistor 30 receives a control voltage at its control input 32. This implementation uses the high output resistance of transistor 30 to minimize the variations in the bias current due to, for instance, changes in the power supply voltage. However, since the biasing current source now is formed by a series arrangement of control transistor 30 and resistor 26, the compliance of this implementation equals the sum of the voltage drops across one of transistors 12 and 14, the voltage drop across control transistor 30 and the voltage drop across resistor 26. The benefits of adding control transistor 30 are now offset by an increase in the power supply voltage that must be supplied to make the circuit function properly. Consequently, in low power supply electronic systems, such as portable battery-operated equipment, the control transistor architecture is undesirable. Also, in high-current applications, such as transmitters, the currents in differential transistor pair 12 and 14 generate such large voltage drops across the transistor pair's loads (not shown) that the voltages across transistors 12, 14 and 30 are reduced towards unacceptably low values, driving these transistors into their saturation range.
FIG. 3 shows a further known implementation of a biasing current source that uses an operational amplifier (op amp) 34 having an inverting input 36 coupled between control transistor 30 and resistor 26, a non-inverting input 38 for receiving a control voltage and an output coupled to the control transistor's control input 32. The op amp 34 impresses the control voltage across resistor 26, rendering the associated current independent of temperature, power supply voltage variations and transistor characteristics. However, the compliance of this circuit is the same as that of the circuit of FIG. 2.